|The Terms Glossary is a service designed to help you understand the welding process and improve your results. The information has been supplied by The Welding Warehouse being reviewed regularly and updated where necessary, however The Welding Warehouse and A & L Supply Company cannot accept liability for inaccuracy of information or any loss incurred as a result of an inaccuracy.
Manual Metal Arc Welding
The oldest of the arc welding processes, Manual Metal Arc (MMA)
welding, has a filler rod coated with flux. When an arc is struck between the
rod and the workpiece the filler rod and flux melt. As the flux melts it
creates a gas shield around the molten weld pool to prevent oxidisation.
Immediately after fusion the flux forms a hard slag over the weld to prevent
oxidisation during cooling. When welding is complete this slag is chipped
off. MMA welding is a versatile process as it can be performed with basic
equipment, used in most conditions and for joining most weldable metals with
the exception of aluminium, which can be MMA welded but not very
Types of Electrode:
- The most common
type of coating, rutile coated electrodes are easy to use, offer good levels
of mechanical strength and produce a weld of good visual appearance.
- Basic, or Low
Hydrogen, coatings are used for higher strength joints and are recommended
for structural applications and materials thicker than 10mm. Basic coatings
are more difficult to use than rutile and do not offer such a good weld
appearance, they are therefore best chosen by those with experience.
- Best known
for deep penetration and rapid burn speed, cellulosic coated rods are
commonly used for professional root welding of plate and pipe.
Welding Current is largely determined by the type of electrode,
the type of job and the material to be welded. Most manufacturers will
recommend a current range for a given product, however as a general guide
allow 35 amps of welding current for every millimetre of electrode diameter
(plus or minus 15% depending on the job) ie: 2.5mm electrode = 88 amps (or a
range of 75 - 102 amps).
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Metal Inert Gas (MIG)/ Metal Active Gas (MAG) welding is the
most popular of the arc welding processes, this is mainly because it is easy
to use and versatile. With an Inert Gas, such as Argon, MIG welding can be
used to weld aluminium. With an Active Gas, such as an Argon/Co2 mixture
(typically 5-20% Co2), MAG welding can be used to weld most steels, including
The process strikes an arc between a continuously fed filler
wire and the workpiece, whilst protecting the arc from oxidisation by pouring
a shielding gas over the weld pool.
Two methods of metal transfer from the filler wire to the
workpiece are used:
Dip transfer - This is achieved at lower voltage settings. The wire is fed into the weld pool creating a
"short circuit" the resulting heat burns the wire off. This happens
many times per second and is responsible for the characteristic crackling
sound of Mig/Mag welding and the associated spatter. Dip transfer offers the
advantage of being able to weld in position, ie vertical.
Spray Transfer - This is achieved at higher voltage settings and is where the filler wire is melted and
transfers in globules to the weld pool. Spray transfer offers smoother welds
with little spatter but cannot be used in position.
Where gas is either unavailable or impractical (such as outside
in windy conditions) flux cored wires can be used. Welding with flux cored
wire is like MMA welding but with a continuous electrode.
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In TIG (Tungsten Inert Gas) welding an arc is struck between a tungsten electrode and the workpiece. A separate filler wire can be added to the weld pool as necessary. The weld pool is protected from oxidisation by pouring an inert gas (usually Argon) over the weld pool. Tig welding is fairly easy to learn and is probably the most versatile of the arc welding processes, the down side is that it is slow and equipment can be prohibitively expensive for all but professional welders.
Tig welding processes fall into two basic categories, DC Output Current is used for most applications including the welding of Steel, Stainless Steel and Copper. AC Output Current is used for Aluminium and Aluminium Alloys.
Tig welding equipment fall into three main types:
DC Only, Scratch/Touch Start - this is the least expensive and most basic form of equipment, relying on the tungsten electrode being touched down onto the job then lifted off to form an arc (much the same as MMA welding).
DC Only, High Frequency Start - this type of equipment uses a burst of high frequency to establish the arc. This is easier and more precise than Scratch starting, but HF start machines are significantly more expensive.
AC/DC, High Frequency Start - these are the most sophisticated, and expensive, of machines and allow the Tig welding of most materials from Steel to Aluminium.
Two types of Tungsten Tig Electrode are available, Thoriated (red tip) for DC applications and Zirconiated (white tip) for AC applications. When AC welding the arc will cause the tungsten to "ball" at the end. The diameter of this ball should not exceed the diameter of the tungsten, if it does a larger tungsten should be used. When DC welding the tungsten should be ground to a point. This point should be as sharp as possible with the grinding lines running from the point, down the length, NEVER around the point.
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Five key hazards should be considered when Arc welding.
Arc-Eye - this condition is caused when the Ultra Violet radiation that arc welding processes emit burns the retina of the eye. Arc-eye is extremely painful and can lead to permanent blindness. It is essential that suitable eye protection is used and that the welding work station is shielded from others.
Skin Burn - Ultra Violet radiation can also burn skin, in exactly the same way as the sun can, only much quicker. Like sunburn, long term implications can include skin cancer. It is therefore essential that suitable workwear is worn and that others are protected.
Fume Inhalation - Welding gives off a variety of particle fumes and gases. As most of the fumes and gases are harmful it is essential that operators protect themselves. This can be done in several ways. Fume extraction offers the best method of protection providing the fume is extracted "at source", that is to say before the fumes can reach the welders breathing zone. Where fume extraction is unavailable or impractical, a suitable respirator should be used to filter the air that the welder breaths. The respirator may be of the disposable, replaceable cartridge or air fed welding helmet type. Where fumes are particularly hazardous, such as when welding galvanised steel and painted materials, both fume extraction and a respirator should be used.
Electric shock - Never use arc welding equipment in damp conditions and always ensure the equipment is in good condition. If in doubt, consult a qualified welding equipment engineer.
Fire - Spatter and sparks from welding can set fire to materials that are left lying around the work station, ie paper, rags etc. Always work in a clean well organised area.
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Porosity - Holes/bubbles in the weld. This is created when gas becomes trapped in the weld pool as it solidifies. Causes include:
Lack of shielding gas - (Mig/Mag & Tig) INCREASE GAS FLOW, gas flow should generally be 8 - 12 litres per minute for Mig/Mag and 12 - 15 litres per minute for Tig.
Too much shielding gas - (Mig/Mag & Tig) If the flow rate, or pressure, of gas is too high turbulence is created. This turbulence allows air to penetrate the shield, this then means you do not have an adequate gas shield. REDUCE GAS FLOW.
Damp electrodes - (MMA). USE DRY ELECTRODES.
Oil/grease on the workpiece - As contaminates burn off they create excess gas, which gets trapped in the weld. CLEAN WORKPIECE before welding.
Surface coating on the workpiece - Paint or plating burning off will create excess gas which gets trapped in the weld. REMOVE SURFACE COATING before welding.
Tall Narrow Welds - Caused by a lack of heat input. INCREASE POWER
Flat Wide Welds - Caused by excessive heat input. REDUCE POWER
Undercut - Caused by excessive heat input. REDUCE POWER
Slag Inclusion - (MMA only) Primary cause is slag from previous weld not being fully removed before second pass is overlaid. Undercut will make slag removal more difficult and increase the risk of slag inclusion. REMOVE ALL SLAG PRIOR TO OVERWELDING.
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Distortion is caused by the localised heat that welding creates along with shrinking of the weld metal as the weld cools. It is difficult to avoid distortion but a few simple steps will help minimise the problem. If possible plan the job to avoid long welds. If long welds are unavoidable use plenty of tacks, weld with as little power as possible and do a series of short runs allowing the workpiece to cool between runs. Using backing plates to help take heat away will also help.
Distortion can be corrected from butt and outside corner welds by peening. Do this by holding a metal block behind the weld and peening evenly along the weld with a hammer. This process stretches the weld metal to correct the shrinking that occurred during cooling. Avoid over peening as this will overstrech the weld and re-distort the workpiece.
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Welding Low Carbon Steels
Low Carbon Steels are an alloy of iron and carbon. Commercial mild steel contains around 0.2% carbon and presents few problems to the welder. Care has to be taken where the carbon content exceeds 0.3% or other alloying elements have been used such as chromium, molybdenum or vanadium. These elements can cause a significant increase in hardness in the area next to the weld (heat affected zone) resulting in brittleness and cracking.
Low Carbon Steels are readily welded using the MMA, Tig or Mig/Mag processes.
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Welding Stainless Steel
Stainless steels are iron based alloys that contain at least 11% chromium uniformly dispersed throughout the metal. The outstanding characteristic of stainless steels is their ability to readily form a chromium-oxide film which acts as a constant buffer against corrosion of the underlying metal. This oxide film is transparently thin, nevertheless it is stable and adherent. If broken or destroyed it reforms instantly and continues to protect the parent metal. Increasing amounts of chromium will provide increased corrosion resistance. Chromium also introduces noticeable physical effects. Steels containing high amounts of chromium develop brittleness. To counter this brittleness and further improve corrosion resistance, Nickel is also added to most common stainless steels. The resulting alloys are tough, but ductile, and offer excellent weldability.
Stainless steel is a poor conductor of heat, this means less power is required to weld it, unfortunately it also means that stainless steel is more prone to distortion. Correct selection of welding consumable is very important. A consumable of the same grade or higher than the workpiece should be used ie, 304L material should be welded with a 308L or 316L consumable while 316L material should only be welded with a 316L consumable. Stainless steel consumables of the 312 grade can also be used for welding dissimilar steels or broken steel parts that are subject to stress or vibration.
Stainless Steel is readily welded using the MMA, Tig or Mig/Mag processes.
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MMA Welding Cracked Cast Iron
Cast Irons are Iron based alloys containing more than 2% carbon. Cast irons fall into four main groups, Grey, Nodular, Malleable and White. These irons can all be considered weldable except White Cast Iron.
Preparation - Preparation is very important in the repair of cracked cast iron. A "U" shaped grove should be ground or gouged into the crack to a depth of 2/3rds the casting thickness, remove all sharp edges (see diagram CI-1). If practical the casting should be pre-heated to 300 degrees centigrade before welding. Heat should be applied slowly and evenly. If pre-heating is not practical the casting should be no colder than room temperature, do not attempt to weld chilled cast iron.
Welding - When MMA welding it is essential that heat input is minimised. This is achieved by keeping weld beads small and "skip welding". Use small electrodes to begin with and never lay down a weld longer than 10 times the electrode diameter in one run,ie 2.5mm electrode = 25mm maximum weld length. Skip weld by welding in different parts of the crack to distribute heat. Take your time, welding cast iron cannot be rushed.
MMA Welding Procedure - Hold the electrode vertical and maintain an arc length of 3-4mm. Use a 2.5mm electrode at around 70 amps to run a weld across each end of the crack. Use a 2.5mm electrode at around 60 amps to run small stringer beads along the "U" preparation (max 25mm long), skip weld in the sequence indicated in diagram CI-2. If the casting is oily or of poor quality this may draw out a lot of contamination, if it does, the stringer beads will have to be ground out and redone until contaminates clear. When all sides of the "U" preparation have been welded the beads can be part ground back if necessary. Use a 3.2mm or 4.0mm electrode to fill the remainder of the "U", remembering to keep the electrode vertical not to break the 10 times electrode diameter rule. Peen weld if necessary while still hot and allow the casting to cool as slowly as possible.
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Welding Tool Steels
Introduction - The weldability of steels with more than 0.2% carbon is generally poor, therefore, Tool Steels with typically 0.3% - 2.5% carbon are difficult to weld and many steel manufacturers actually discourage welding. However, it is a viable proposition if carried out properly and can have considerable economic impact when viewed against the cost of producing a new tool.
General Information - Tool Steels containing 0.3% - 2.5% Carbon, as well as alloying elements such as Manganese, Molybdenum, Chromium, Nickel, Tungsten and Vanadium, are adversely affected by localised heat input due to their high hardenability. In air, welds cool quickly once the heat input is removed, allowing Martensite to form, this renders the weld and Heat Affected Zone unacceptably hard, this in turn causes stress, which can lead to cracking.
The following is a general description of the Welding Equipment, Consumables and Techniques required in order to successfully repair Tool Steels. The skill and experience of the Welder is, of course, important, but Tool reclamation can, and is, carried out with great success by Artisans who are not primarily Welders.
Welding of tooling may be required for a number of reasons: Chipped or worn cutting edges, repair of cracked or worn-out tooling, correction of machining errors, design changes.
Welding Methods - MMA is generally the most convenient method of welding in terms of equipment as all that is required is a power source (AC or DC) and a consumable electrode of a composition compatible with the Tool Steel to be welded. The drawback is that it is difficult to achieve a deposit with zero undercut and that there is the need to de-slag between each run. There is also a need to stock several diameters of electrode to cope with the various repairs that may arise. Tig welding is probably the most widely used method for weld repairing tooling. Tig is particularly suited to repairing worn or chipped cropping edges, as it is extremely precise, no slag is formed during welding and the size and shape of the deposit can be finely controlled. With the correct parameters, undercut can be eliminated, thus avoiding the need for excess weld deposit and resultant finishing. The Mig/Mag process is perhaps the most user-friendly of the processes, however, filler materials are not as readily available as for Tig and Mig/Mag is not as precise.
Filler Metal Charateristics - The chemical composition of the weld deposit is determined not only by the analysis of the consumable, but also the composition of the base material and the mixing of the two during welding (dilution). As dilution is a variable, dependant upon arc parameters, it is necessary that the filler material is compatible with the base material and gives a hardness that approximates to that required. The final hardness of the weld itself will depend largely upon cooling rate and dilution factor.
The weld properties should satisfy the following requirements: Uniform composition, hardness and reaction to any heat treatment. Homogeneity. Hardness, surface finish and wear resistance to match the tool in question.
For the three main Tool Steel applications (Cold Work, Hot Work and Plastic Mould), the most important filler metal characteristics are:
Cold Work Tools - Hardness, Toughness, Wear Resistance.
Hot Work Tools - Hardness, Temper compatibility, wear resistance, Hot Cracking Resistance.
Plastic Mould Steel - Hardness, Wear resistance, Corrosion resistance, Ability to take a polish.
Pre Heating Requirements - The main reason for pre-heating a tool-steel and welding at an elevated temperature stems from the high hardenability, and therefore susceptibility to cracking, of Tool Steel Welds and Heat Affected Zones. By pre-heating and maintaining that temperature, the cooling rate is slowed down considerably, this minimises the chance of brittle Martensite forming. It also allows Hydrogen to diffuse out of the weld zone. Retained Hydrogen is one of the major causes of cracking in a highly alloyed material.
Welding Proceedure - The successful welding of Toolsteel is unlikely to be achieved unless the correct procedure is rigidly adhered to, particularly with regard to joint preparation, pre-heat requirements and post weld heat treatment (or cooling rate). Joint preparation is paramount. Cracks must be completely removed, leaving angled sides and a radiussed bottom, this should be at least 1mm wider than the largest consumable to be used. Verification of crack elimination can be determined by MPI or Dye Penetrant methods, remove all contamination by grinding. Joint surfaces are clad using small diameter electrodes or TIG Wire of similar composition to the base material, alternativly, a softer "buttering" layer which serves to isolate the weld from harmful "pickup" may be applied. Multi-runs are used to complete the overlay to ensure that beads, initially deposited, are annealed by subsequent runs. Final runs are built up well above the surface of the Tool. Even small welds should consist of a minimum of two runs deposited as stringer beads, (ie No Weave).
Case Hardening Steels - Case-hardening steels, including those that are Nitrided, present a particular problem when welding due to the different reaction to localised heat of the hard, thin case compared with that of the bulk of the material. The only satisfactory method of dealing with this is to completely remove the case from the weld zone by grinding. The case material is then replaced by a consumable of the desired hardness.
Cast Iron Dies - The wear on cast iron dies usually takes place on draw edges. These can be successfully reclaimed, usually in a cold condition, by the use of a consumable to isolate the free carbon in the cast iron, and then a wear-facing material to give the desired properties. It is not unusual to use cobalt-based alloys (Stellite) or Nickel Aluminium Bronze in these situations to give a deposit with a low coefficient of friction coupled with wear resistance.
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